1. Field of the Invention
This invention relates to a heat pump control and more particularly to a control system for coordinately regulating the defrost operation of a dual compressor heat pump system.
2. Description of the Prior Art
The utilization of a dual compressor heat pump is advantageous for being able to independently stage the compressors to control the energy input required for necessary cooling and heating operations. The term heat pump as used herein refers to a reversible refrigeration system capable of delivering on demand either heating or cooling to a region to be conditioned. In most smaller heat pump systems, a single compressor is employed. Control of these single compressor systems is relatively simple and presents few problems. However, in many larger heat pump systems two compressors are utilized with each compressor arranged to pump refrigerant through an associated closed loop circuit.
In heat pump systems using two compressors, it is the common practice to stage the operation of compressors when the heat pump is in the cooling mode of operation whereby the compressors are brought into operation in sequence as the cooling load of the system increases. However, both compressors are normally operated when the system is providing heating to the air conditioned region without regard to the heating demands placed on the system. The operation of both of the compressors in the heating mode is carried out primarily to prevent an inadvertent cycling load on the compressors when the system is undergoing a defrost cycle. As is well known in the art starting one of the compressors when the outdoor fan is off as is typical during defrost will force the system to operate under adverse conditions which could damage the system.
The continuous operation of both compressors to avoid the problems associated with defrosting, however, gives rise to other problems which, although not as dramatic, can also lead to needless wasting of energy and eventual failure of the system. In United States patent application, Ser. No. 739,398, now abandoned entitled, "Two Stage Compressor Heating" assigned to the assignee hereof and having the same inventors as herein, there is disclosed a heat pump control system for staging the operation of the dual compressor system in the heating mode of operation. Therein is shown an electrical circuit involving a defrost system wherein one compressor or two compressors may be operated to meet the heating load as sensed by a thermostat. Therein it is disclosed that when defrost is necessary both outdoor heat exchangers will be simultaneously defrosted. By averaging the refrigerant temperatures in each system the necessity of defrost is determined. If only one compressor is in operation, then the other compressor will be energized such that both operate in a cooling mode when defrost is required.
The present system concerns itself with the staged operation of a dual compressor system in the heating mode of operation as well as independent defrost of the separate outdoor heat exchangers. The electrical control circuit provided energizes the second compressor when the first compressor is in a defrost cycle such that heating is supplied to the region to be conditioned notwithstanding that the second compressor is operated in the cooling mode to defrost the outdoor heat exchanger. Furthermore, individual relay contacts are provided in each defrost system such that if either of the compressors is being operated in a defrost cycle, the other compressor may not commence its defrost cycle. Consequently, in the heating mode of operation one compressor is always supplying heat to the enclosure or region to be conditioned notwithstanding the mode of operation of the other compressor.